How can I replicate function of relative transform of spring arm/camera boom?

I’m talking about getting the relative rotation from a camera boom by using the boom as the target. It seems others have posted with people posting various suggestions but nothing very definitive has been proposed as an alternative. If what had existed prior to 4.17 and 4.18 was fixed with a bug, how can we replicate those values? The way I had it arranged and used previously worked great in 4.16, but updating to 4.18 has stopped this from working and stopped large segments of a project from working.

How can I get the same values, or what exactly was this reading? (parent rotation - boom rotation?) How can I replicate this functionality? Any help would be greatly appreciated to reproduce what functionality was lost with updates.

The image linked above showing the node was shared previously in this thread.